There is often a need to determine whether an ambient, enclosed cavity or component body temperature has crossed a predetermined temperature threshold. Numerous electrical circuits variously utilizing electromechanical and discrete temperature sensitive components, including bi-metallic strips and thermistors, have been developed to meet this need. While such solutions are conventionally utilized, they are not the best possible solution. Rather, such solutions are utilized at the cost of having to redesign the sensing circuit with each application. The detracting consequences, therefore, include repeated circuit design efforts and the corresponding product development complexity, the fact that the circuits must often involve mechanical and electromechanical design and implementation considerations, and the number of components and printed circuit board area consumed to realize the necessary circuit.
An entirely electronic circuit, and one that could be substantially implemented as a single monolithic integrated circuit would therefore be desirable. There are, however, a number of significant problems to achieving this goal. Conventional thermistors are often used to provide a temperature dependent voltage. Although well-characterized, both in operation and in manufacture, they are not suitable for monolithic fabrication due to their size, construction composition and sensitivity to production process variations.
Other temperature dependent elements more appropriate for monolithic fabrication are known to exist. For example, the base to emitter voltage drop of a bipolar transistor at a fixed current flow is known to vary directly with the temperature. The nominal voltage drop, V.sub.BE, however, is highly dependent on the parameters of its fabrication process even as between ostensibly identical fabrication runs.
Further, once fabricated, there is no practical manner of trimming the resultant V.sub.BE value so as to achieve a standardized circuit behavior. Finally, the total change in the voltage V.sub.BE over even a full range of temperatures, such as from 25.degree. to 175.degree. Centigrade is quite small. Therefore, the circuit required to support the bipolar transistor temperature sensor must itself be insensitive to temperature while being quite sensitive to the V.sub.BE voltage changes in order to yield an acceptable operating tolerance.